The guest post by a former member of the CIA’s Senior Intelligence Service, John Sipher, paints the Steele Dossier as mostly but not utterly reliable:

Although the reports were produced episodically, almost erratically, over a five-month period, they present a coherent narrative of collusion between the Kremlin and the Trump campaign. As a result, they offer an overarching framework for what might have happened based on individuals on the Russian side who claimed to have insight into Moscow’s goals and operational tactics. Until we have another more credible narrative, we should do all we can to examine closely and confirm or dispute the reports.

Key caveat:

I spent almost thirty years producing what CIA calls “raw reporting” from human agents. At heart, this is what Orbis did. They were not producing finished analysis, but were passing on to a client distilled reporting that they had obtained in response to specific questions. The difference is crucial, for it is the one that American journalists routinely fail to understand. When disseminating a raw intelligence report, an intelligence agency is not vouching for the accuracy of the information provided by the report’s sources and/or subsources. Rather it is claiming that it has made strenuous efforts to validate that it is reporting accurately what the sources/subsources claim has happened. The onus for sorting out the veracity and for putting the reporting in context against other reporting – which may confirm or deny the new report – rests with the intelligence community’s professional analytic cadre. In the case of the dossier, Orbis was not saying that everything that it reported was accurate, but that it had made a good-faith effort to pass along faithfully what its identified insiders said was accurate. This is routine in the intelligence business.

That said, however,

As outsiders without the investigative tools available to the FBI, we can only look at the information and determine if it makes sense given subsequent events and the revelation of additional information. Mr. Steele did not have the benefit of knowing Mr. Trump would win the election or how events might play out. In this regard, does any of the information we have learned since June 2016 assign greater or less credibility to the information? Were the people mentioned in the report real? Were their affiliations correct? Did any of the activities reported happen as predicted?